As reported, for example, in Prota, Progress in the Chemistry of Melanins and Related Metabolites, Med. Res. Reviews, 8:525-56 (1988), melanins are naturally occurring pigments present in hair and skin. In humans, biosynthesis takes place in tyrosinase containing melanocytes. The tyrosinase enzyme catalyzes the hydroxylation of tyrosine to dopa and its subsequent oxidation to dopachrome. Once formed, dopachrome undergoes a series of complex reactions in the formation of eumelanins and phaeomelanins.
Melanins provide black and deep brown pigments, and are formed by oxidative polymerization of 5,6-dihydroxyindole derived biogenetically during the melanogenesis. On the other hand, phaeomelanins provide yellow to reddish brown pigmentation to hair and are formed by oxidative polymerization of cystein-S-yl-dopas via 1,4-benzothiazine intermediates.
Synthetic 5,6-dihydroxyindole (DHI) has been disclosed in the prior art for use in hair and skin dyeing. For example, U.S. Pat. No. 2,934,396 to Charle discloses a process for dyeing hair by contacting hair with an aqueous solution of DHI having a pH of at most 7 for 5 to 60 minutes, followed by an application of an aqueous solution capable of inducing oxidation and/or polymerization of DHI.
Dopa and dopamine are disclosed as hair dyeing precursors in the process of Herlihy, U.S. Pat. No. 4,746,322, wherein the aqueous hair dyeing composition comprises said precursor, an organic compound to assist dye dispersion and an iodate or periodate oxidant.
Interest in melanogenesis to dye hair is quite high. This is because synthetic melanin pigments provide an exceptionally natural-looking deep brown or black color. Moreover, they are not irritating to the skin, nor are they mutagenic.
The discovery of the formation of melanin pigments for permanent hair coloration by oxidation with ferricyanides has proved to be very useful. It has now been found, quite suprisingly, that the results achieved with the ferricyanide oxidation can be appreciably improved by a post-oxidation treatment to increase melanin formation.
The hair dyeing process of the present invention contemplates as an initial step the preparation of an aqueous hair dyeing composition by reacting a dopa species as hereinafter defined, with a ferricyanide oxidant to form a melanin-forming hair dye precursor, and applying the aqueous composition to the hair. The melanin precursor contained in said aqueous composition is capable of diffusing into the hair shaft in an amount effective to dye hair permanently upon its coincident conversion to melanin while in the hair.
The aqueous hair dyeing composition is produced by initiating reaction between the dopa species or a salt thereof with an inorganic oxidant that is a soluble ammonium, alkali or alkaline earth metal salt, especially a sodium or potassium salt, of ferricyanide in an aqueous reaction medium buffered by sufficient buffering agent to maintain the reaction medium pH from about 6 to about 11 throughout the series of reactions that take place leading to the melanin precursor.
In order to achieve the permanent dyeing of hair with a ferricyanide oxidant, it is critical to generate melanin from the melanin-forming hair dye precursor in the aqueous hair dye composition in such amount as to effect a color change to the hair. It is further critical that the hair dye composition be applied to the hair prior to the substantial formation of melanin so that the melanin precursor formed during the reaction may diffuse into the hair prior to the generation of melanin, the melanin then being formed within the hair.
In the case in which the dopa species is dopa or a salt thereof, the reaction with the oxidant leads to the formation of 5,6-dihydroxyindole, which melanin precursor, upon its conversion to melanin, provides hair with a permanent black color. In the case of other dopa species, melanin precursors are obtained which, upon conversion to melanin, produce a range of shades depending upon the selection of the substituted dopa compound.
A further aspect of the ferricyanide oxidation process is the optional incorporation of an oxidative hair dye component selected from the group consisting of direct dyes, primary intermediates, couplers and mixtures thereof in the reaction mixture. Following the initial dopa species-oxidant reaction, it is believed that the direct dyes, primary intermediate(s) and/or coupler(s) present in the reaction mixture react at least in part with the intermediate compounds formed prior to the melanin precursor, thereby providing chromatic characteristics to the melanins ultimately obtained.
In another aspect of the ferricyanide oxidation method, the rate of formation of indolic melanin precursors such as 5,6-dihydroxyindole is hastened by proper selection and amount of the buffer, apart from its requirement for maintaining pH of the reaction medium. Preferably, the buffer is a phosphate, carbonate or bicarbonate, and typically is included in substantial excess over the amount needed to maintain the requisite pH.
It has also been observed that the rate of melanin formation improves by treatment of the hair with agent(s) that promote melanin formation, e.g., a solution of a metal ion salt.
It has now been found that the ferricyanide process in all of its variations can be improved by a post-oxidative treatment of the melanin dyed hair utilizing, preferably sodium periodate.
The improved process of the present invention may conveniently be practiced by providing premeasured amounts of the reactants in separate containers packaged in kit form. The user simply admixes the reactants and utilizes the mixtures in accordance with the procedures described herein. No special expertise is required to carry out the process, and accordingly the product and process are equally suitable for in-home use by the nonprofessional as well as salon use by the professional. Advantageously, the product in kit form is shelf-stable and is therefore suitable for retail sale and without precautions generally required for melanin-forming precursors, such as 5,6-dihydroxyindole, e.g., storage under anaerobic conditions.